In long distance optical fiber transmission systems, it is desirable to launch as high an optical power as possible, enabling the lightwave signals to be transmitted without the need for additional components such as repeaters and amplifiers, which increase the cost of communication systems. However, the combination of high powered, narrow linewidth optical sources with low-loss single mode transmission fiber opens the possibility of signal degradation and increased bit error rates, attributable to a host of nonlinear fiber-related phenomena that have been previously considered inconsequential. These nonlinear phenomena include stimulated Brillouin scattering (SBS), stimulated Raman scattering, self-phase modulation and, if two or more optical channels are involved, cross-phase modulation and four-wave mixing.
Stimulated Brillouin scattering within a fiber results from photons being scattered by localized refractive index variations induced by acoustic waves. These refractive index variations are caused, in particular, by ultrasonic vibrations in the glass lattice that makes up the fiber core. Furthermore, owing to the dependence of refractive index on light intensity in the nonlinear regime, the presence of intense light in the fiber will also induce lattice vibrations, which in turn induce sound waves that then scatter more light. Ultimately, light from an intense forward propagating signal (referred to as a “pump” signal) can provide gain for a backward propagating or “Stokes” signal. This scenario is a classical description of SBS. SBS threshold power (denoted as PSBS) is arbitrarily defined as the level of input optical pump signal power (Ppump) at which the power of the backward Stokes signal (PStokes) becomes equal to Ppump at the fiber input. SBS threshold power increases with the linewidth of the light being propagated along a fiber. For this reason, concern over the adverse effects of SBS was minimal—until the introduction of narrow linewidth laser sources. As narrow linewidth sources become more readily available, and as such lasers are likely to be the optical source of choice for future optical fiber transmission systems, SBS has the potential for significantly contributing to signal degradation at relatively low input power levels.
To date, several techniques have been demonstrated to suppress the SBS in optical transmission systems. In general, these techniques fall into two main categories: (1) modifications of the fiber media to reduce SBS; or (2) modifications of the laser source to alter the linewidth. In the first category, it is possible to influence the refractive index (and acoustic velocity) along the longitudinal direction of the fiber, thus varying the Brillouin gain profile along the fiber. This variation avoids the accumulation of gain within a small bandwidth and results in a broader gain profile and high SBS threshold. However, this fiber modification approach is not practical, since it is relatively difficult to introduce these effects into the fiber as it is manufactured and, more importantly, cannot be used to reduce SBS on the extensive embedded base of the optical fiber network already in place.
The alternative solution of modifying the laser source to affect the level of SBS has been found to be more practical and can be used with existing optical fiber systems. This technique is based on the property of broadening the laser linewidth by means of modulation. U.S. Pat. No. 5,329,396, issued to D. A. Fishman et al. on Jul. 12, 1994, discloses one exemplary prior art arrangement which impressed FM modulation on the laser bias current to increase the linewidth. This direct FM approach, by using a dither signal, provides a relatively large frequency excursion (on the order of, for example, 10 GHz). By means of this technique, the SBS threshold has been increased as much as 15 dB. However, impressing an FM signal on the laser bias has been found to also result in substantial AM (defined as “residual AM”), degrading system performance. Alternatively, external phase modulation (PM) can be used to broaden the laser linewidth. U.S. Pat. No. 5,166,821 issued to D. Huber on Nov. 24, 1992 discloses one such PM arrangement. However, in optical transmission systems, this external PM technique typically degrades the dispersion characteristics of the signal due to an excessive increase in the linewidth of the laser source.
Thus, a need remains in the art for an arrangement that reduces SBS without introducing other types of signal degradation in the optical transmission system.